In recent years, electrically-powered vehicles mounted with an electric motor for driving the vehicle, such as hybrid vehicle, electric vehicle, fuel cell vehicle and the like have been of interest as environmentally-friendly vehicles. In the electrically-powered vehicle, when an output of the vehicle-driving electric motor is used to drive the drive wheels, a periodically varying component may occur to the rotational speed of the electric motor to thereby cause vibrations of the vehicle. Such a phenomenon, for example, can occur when the vehicle accelerates or decelerates.
Japanese Patent Laying-Open No. 2006-136184 (PTL 1) discloses motor control for reducing such vehicle's vibrations. Specifically, the periodically varying component of the rotational speed of the vehicle-driving electric motor is extracted and a vibration-reduction torque of the opposite phase relative to the extracted variation component is added to a torque command value to thereby implement the aforementioned control for reducing vibrations. It should be noted that PTL 1 discloses that this vibration-reduction control is performed only when pulse width modulation (PWM) control (particularly sine wave PWM control) which provides a quick control responsiveness is applied.
As for the hybrid vehicle mounted with an engine in addition to the vehicle-driving electric motor, the engine may be operated intermittently depending on the state of the vehicle. When the engine is started or stopped while being intermittently operated, speed variations of the vehicle can cause vehicle's vibrations. Japanese Patent Laying-Open No. 2009-33947 (PTL 2) discloses that a hybrid vehicle undergoes the vibration-reduction control which is performed when its engine is started/stopped. According to PTL 2, the magnitude of the torque of the electric motor is changed so that it is appropriate for the engine's cycle such as expansion stroke—compression stroke for example, to thereby implement the vibration-reduction control for alleviating mechanical vibrations of the engine. PTL 2 also discloses that the vibration-reduction control is performed only when the PWM control (sine wave PWM control) is applied.